The present invention relates to a control device for vehicle speed which is adapted to memorize the speed of a vehicle and automatically maintain the vehicle at the memorized vehicle speed, and more particularly to a control device for vehicle speed which has a function capable of memorizing the vehicle speed in a desired state while accelerating the vehicle in accordance with switch actuation.
The control device for vehicle speed of this kind generally comprises a throttle driving means operatively coupled to a throttle valve for driving the same, a vehicle speed detecting means for detecting the vehicle speed, a vehicle speed memorizing means for memorizing the vehicle speed, an electronic control means for comparing the memorized vehicle speed with the current vehicle speed to control the throttle driving means so that the difference therebetween becomes zero, and at least one command switch. The command switch includes a resume switch. In such control device, when the resume switch continues to be pressed over a certain time, it comes into an acceleration/memorization mode. More specifically, during the time the driver is pressing the resume switch, the vehicle is automatically accelerated, and when the switch is released, the vehicle speed at that time is memorized and control is made to keep the memorized vehicle speed as a target vehicle speed.
Meanwhile, a driving mechanism of the vehicle has been set in an acceleration mode up to the time immediately before the resume switch is released. Such a driving mechanism, i.e., engine system and control system, has a relatively large delay in its response because of a mechanical system included therein. This response delay is large particularly in the vehicle loaded with a turbocharger. Accordingly, when an electronic device for vehicle speed control is changed in its output from an acceleration mode to a target speed holding mode upon releasing of the resume switch, it takes a fairly long time for the vehicle to change from an acceleration mode to a target speed holding mode in practice. As shown in FIG. 1, hence, there causes an undesirable phenomenon that is called overshoot i.e., the vehicle speed is first increased beyond the memorized vehicle speed and then lowered down to the memorized vehicle speed for a certain time.
For this reason, there has been proposed a technique intended to reduce the apparent overshoot by memorizing the vehicle speed a little higher than that at the moment the resume switch is released, as shown in FIG. 2. But this technique is unsatisfactory because the vehicle comes into constant-speed traveling at a vehicle speed different from one that the driver desires. As an alternative, there has been proposed another technique intended to hold acceleration at a relatively small constant value during press of the switch. But with this technique, it takes a longer time for the vehicle to reach the desired vehicle speed.